1. Field of the Invention
This invention relates in general to communication devices and in particular to communication devices incorporating a receive signal path correction system with self-calibration capabilities.
2. Description of the Related Art
Gain and phase imbalances between the I and Q quadrature channels of a receive signal path of a communication device occur because of inherent circuit level mismatches between analog circuits in these quadrature signal paths. This can significantly degrade the detectability of the received signal under both static as well as multipath fading channel conditions for both Zero Intermediate Frequency (IF) as well as Very Low Intermediate Frequency (VLIF) receivers. In addition, in the case of VLIF receivers, the performance of these receivers is highly dependent on the suppression of the image signal. Suppression of the image signal in such receivers can be significantly improved by eliminating these gain and phase imbalances.
Traditionally, it has been difficult to achieve such I/Q performance without performing open loop factory calibration steps. This is unfortunately an expensive approach as it adds to production cost and time. Even if factory calibration is performed, it is difficult to preserve the factory correction performance over temperature, supply voltage, and channel frequency variations over the life of the wireless product. This is because after a one-time factory correction is performed to correct for the specified imbalances, these imbalances can vary over temperature, supply voltage, and channel frequencies.
FIG. 1 illustrates a block diagram of a conventional receive signal path 10 within a communication device. As illustrated a complex communication signal 12 is received by the communication device and de-interleaved by a demodulator 14 into an I-channel received signal 16 and a Q-channel received signal 18. The demodulator 14 typically includes one or more quadrature receive mixers 20,22 receiving inputs from a conventional local oscillator 24. Typically the quadrature receive mixers 20,22 convert the complex communication signal 12 to the I-channel received signal 16 and the Q-channel received signal 18, which are baseband signals, using the conventional local oscillator 24. The I-channel received signal 16 is then processed through an I-channel post mixer amplifier (PMA) 26, which provides programmable gain to amplify the baseband signal. Next, the amplified signal is processed through a conventional I-channel anti-aliasing filter (AAF) 30, which provides attenuation to out of band frequencies. Next, a conventional I-channel analog to digital converter (A/D) 32 converts the signal from an analog format to a digital format, to produce a processed I-channel signal 34. Similarly, the Q-channel received signal 18 is processed through a Q-channel post mixer amplifier (PMA) 36 which provides programmable gain to amplify the baseband signal. The amplified signal is processed through a conventional Q-channel anti-aliasing filter (AAF) 40, which provides attenuation to out of band frequencies. Next, a conventional Q-channel analog to digital converter (A/D) 42 converts the signal from an analog format to a digital format, to produce a processed Q-channel signal 44.
Potential sources of I/Q gain and phase imbalances in the receive signal path 10 of a communication device include the quadrature receive mixers 20,22, the I-channel post mixer amplifier 26, the Q-channel post mixer amplifier 36, the conventional I-channel anti-aliasing filter 30, the conventional Q-channel anti-aliasing filter 40, the I-channel analog-to-digital converter 32 and the Q-channel analog-to-digital converter 42. The total gain imbalance due to these circuits can exceed 4 dB over process, temperature, and supply voltage variations. For phase modulated spread spectrum communications protocols, such as WCDMA, it is desired that this gain imbalance be controlled to less than 0.8 dB to preserve BER performance under static and multipath fading conditions. For amplitude-modulated systems, the gain imbalance requirements are even more stringent.
In addition, the dominant sources of I/Q phase imbalances are due to the quadrature receive mixers 20, 22, the conventional I-channel anti-aliasing filter 30, and the conventional Q-channel anti-aliasing filter 40. The conventional local oscillator 24 causes the majority of the phase imbalance at the output of the quadrature receive mixers 20, 22. The total phase imbalance due to these circuits can exceed 10 degrees over process, temperature, supply voltage, and channel frequency variations. For phase modulated systems such as Code Division Multiple Access (CDMA) and Global System for Mobile Communications (GSM)/Enhanced Data rates for GSM Evolution (EDGE), it is desired that this phase imbalance be controlled to less than 3 degrees to preserve BER performance under static and multipath fading conditions.
Therefore, what is needed is a high performance, low cost and low power system for correction of I/Q quadrature gain and phase imbalances in the receive signal path of a communication device.